A Legionella result tells you two things you usually forget to ask about: how good the sample was, and how long ago the water it came from actually existed. By the time a culture result lands, the water it describes is a fortnight old and long gone down the drain. The lab cannot rescue a badly taken sample, and it cannot make a slow bacterium grow faster. Understanding what happens in those two weeks is the difference between commissioning tests that mean something and paying for numbers you can’t use.

This is not about doing the lab’s job. It’s about sending the lab something worth testing, and reading the answer for what it really is.

What the lab actually does with your bottle

Culture is the long-standing reference method for Legionella in water, and it is deliberately patient because the organism is slow-growing [1]. Stripped of the jargon, the journey looks like this.

Your sample arrives and is logged. The lab concentrates it — Legionella is often present in small numbers, so the sample is typically filtered or spun down to gather any bacteria from a larger volume into a small one. That concentrate is then spread onto a selective growth medium: a nutrient agar dosed to favour Legionella and suppress the faster-growing organisms that would otherwise swamp the plate. Some portions may get a heat or acid treatment first, again to knock back competitors that grow more readily than Legionella does.

Then everyone waits. The plates incubate for days while any Legionella present forms visible colonies. Suspect colonies are picked off and confirmed — checked that they really are Legionella and, where it matters, identified to species and serogroup, since Legionella pneumophila serogroup 1 is the type most associated with human illness. Finally the colonies are counted and scaled back up to the original volume, which is why your report reads as colony-forming units per litre (CFU/L) rather than a raw number on a plate.

Start to finish, that commonly runs to around 7–14 days, with a longer tail on positives that need full confirmation. Treat the exact turnaround as something to confirm with your lab rather than a fixed figure [1].

Why it takes two weeks when a swab test takes minutes

This is the question that frustrates every facilities team, and the honest answer is biology. Culture waits for living cells to multiply into countable colonies, and Legionella simply grows slowly. You cannot compress that.

The alternative people reach for is PCR, which detects Legionella DNA and can return a result far quicker. The trade-off is real and worth understanding before you choose: PCR finds genetic material whether the cells are alive or dead, so it can flag a system that culture would clear, and it answers a slightly different question. Neither is “better” in the abstract — they suit different jobs, and Detecting Legionella: culture and PCR testing methods sets the two side by side. The point for now: culture is slow because it insists on growing live organisms, and that insistence is also its strength.

Send the lab a sample worth testing

Most poor results trace back to the sample, not the science. The lab can only analyse what reaches it, in the state it reaches it. Before anyone goes out with a bottle, run the sampling against this list — and keep it consistent every time, because a sample taken differently on each visit gives you trends you can’t trust.

  • Use the lab’s sterile bottles, with neutraliser where the supply is chlorinated. Residual disinfectant keeps acting on the bacteria in transit; the neutraliser stops the clock so the result reflects the water, not the journey. Confirm volume and bottle type against BS 7592 and the lab’s instructions [2].
  • Decide pre-flush or post-flush deliberately, and record which. A first-draw sample tells you about the outlet and its local pipework; a flushed sample tells you more about the incoming system. They answer different questions — pick the one your risk assessment needs and note it.
  • Sample cleanly. Don’t touch the bottle neck or rim, don’t rinse the bottle, and fill to the line. Contamination from hands or the tap surface is a classic false signal.
  • Label so the result is traceable. Outlet identifier, location, date, time, temperature at sampling, and pre/post flush. A CFU/L with no context is almost useless when you try to act on it weeks later.
  • Get it cold and quick. Keep samples cool and move them to the lab promptly; long, warm transit lets the population drift before analysis even starts.
  • Use an accredited lab and keep the chain of custody. Accreditation (UKAS, for the relevant test scope) is what lets you stand behind a result if it is ever challenged — confirm the lab’s accredited scope covers Legionella water testing [4]. Choosing a UKAS-accredited lab for Legionella testing covers how to check that properly.

Reading the result without over-reading it

When the report comes back, resist two opposite errors: treating a clean result as a clean bill of health, and treating a positive as an instant emergency.

A culture count is a snapshot of the sampled outlets at one moment, filtered through how well the sample was taken. A negative describes those outlets on that day; it does not certify the system, and it never replaces control of temperature, stagnation and cleanliness. A positive tells you Legionella was growing somewhere in that water, but what the number means — and what action it triggers — depends on the count, the species, the system and your scheme, which is its own subject in Interpreting Legionella test results and counts.

Worth stating plainly: this is general background to help you commission and read tests, not a method statement or clinical guidance. Sampling supports verification and investigation; it does not run your control programme, and how often you sample follows the system and the risk assessment rather than a fixed calendar [3].

FAQ

Why does Legionella culture take so long when other tests are quick?

Because culture grows living bacteria into visible colonies, and Legionella is slow-growing. The incubation takes days, plus time to confirm and identify suspect colonies [1]. Quicker methods like PCR detect DNA instead of waiting for growth — faster, but answering a different question.

Can a rapid or on-site test replace sending a sample to the lab?

Not as a like-for-like substitute. Faster methods can help with screening or early warning, but culture remains the reference method for a countable, accredited result. Treat a rapid test as a prompt to investigate, not the final word — and check what your risk assessment and lab actually require.

What does “CFU per litre” mean on my report?

Colony-forming units per litre — roughly, how many viable Legionella the lab grew from your sample, scaled back to the original volume. It measures what was culturable in that sample, not a direct health threshold. Interpreting a given count belongs with your responsible person and your scheme.

Sources

[1] CDC, “Laboratory Testing for Legionella”. https://www.cdc.gov/legionella/php/laboratories/index.html [2] BSI, “BS 7592:2022 - Sampling for Legionella bacteria in water systems. Code of practice”. https://knowledge.bsigroup.com/products/bs-7592-sampling-for-i-legionella-i-bacteria-in-water-systems-code-of-practice-1 [3] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [4] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm